Recently, in view of global environmental protection, attention has been focused on biodegradable polymers that are decomposed under natural environment by the action of microorganisms in soil or water, and various biodegradable polymers have been developed. Among such polymers, for example, polyhydroxybutyrate, polycaprolactone, aliphatic polyester composed of an aliphatic dicarboxylic acid component such as succinic acid or adipic acid and a glycol component such as ethylene glycol or butanediol, and polylactic acid are known as biodegradable polymers capable of being melted and molded.
In particular, polylactic acid, which is relatively inexpensive and has good heat resistance with a melting point of about 170° C., is a promising biodegradable polymer capable of being melted and molded. In addition, the monomer lactic acid has been recently produced at lower cost by fermentation process using microorganisms and, therefore, polylactic acid has been capable of being produced at lower cost. Thus, polylactic acid has been studied to be used as a general-purpose polymer as well as a biodegradable polymer.
Unfortunately, polylactic acid has heat resistance lower than that of petrochemical polyester such as polyethylene terephthalate resin or polybutylene terephthalate resin, and has a problem in which fabrics made of it cannot be ironed.
Lactic acid has optical isomers, and it is known that a stereocomplex crystal formed of a mixture of poly-L-lactic acid and poly-D-lactic acid, which are polymers of L-lactic acid and D-lactic acid, respectively, has a melting point higher than that of a crystal of poly-L-lactic acid or poly-D-lactic acid alone. Unfortunately, a composition obtained by simply compounding poly-L-lactic acid and poly-D-lactic acid contains not only a stereocomplex crystal but also crystals of poly-L-lactic acid itself and poly-D-lactic acid itself as residual components. A complete, high-melting-point, stereocomplex has not been obtained yet, and the heat resistance obtained has been low.
There are disclosed a method of performing stretching to twice or more in at least one direction for the purpose of increasing the stereocomplex crystal content and the heat resistance (Japanese Patent Application Laid-Open (JP-A) No. 2007-204727), a method of performing a heat treatment in the temperature range from the melting point derived from a homo-crystal of polylactic acid to less than the melting point derived from a stereocomplex crystal (JP-A No. 2008-63356), and a method in which a diblock copolymer of poly-L-lactic acid and poly-D-lactic acid is used (JP-A No. 2008-248022).
Unfortunately, the melting peak derived from a stereocomplex crystal produced by these techniques has a wide half value width, and these techniques have been not able to produce a complete stereocomplex, although they can increase the stereocomplex crystal content. Therefore, these techniques have not been able to improve heat resistance, chemical resistance, and mechanical properties sufficiently.
The conventional techniques described above also have a problem in which they need a post-process such as stretching, heat treatment, or polymerization and therefore increase the production cost.
Therefore, adding a nucleating agent to poly-L-lactic acid and poly-D-lactic acid has been studied. JP-A No. 2003-192884 discloses a method of forming a stereocomplex by adding a phosphate metal salt. In this method, crystals of poly-L-lactic acid itself or poly-D-lactic acid itself are partially left, and a complete, high-melting-point, stereocomplex has not been obtained.
It could therefore be helpful to provide a polylactic acid stereocomplex having good heat resistance and a high level of mechanical properties and chemical resistance, a method for efficient production thereof, a nucleating agent for polylactic acid resin containing such a polylactic acid stereocomplex, a method for producing a polylactic acid resin composition using such a nucleating agent, and a molded product.